This invention relates to the field of use of digital video capture. More particularly, the present invention relates to method and apparatus to minimize flicker effects from a discharge light source during digital video capture.
Digital cameras are currently being used in many applications, including both still image and video acquisition. To acquire images, digital cameras utilize a sensor array made up of an arranged pattern of photodiodes (e.g., light sensitive diodes, or, photosensors). Each photodiode measures the amount of light it receives by storing a corresponding amount of charge on an integrated capacitor. The amount of charge stored by each photodiode is then converted into a digital value by an analog-to-digital converter such that all the digital values, after being converted and reassembled into a particular array, can be processed to arrive at a digital image.
Typically, the photosensor array is exposed to the scene to be captured through the use of either a mechanical or electronic shutter that allows either (1) light to fall upon the photosensor array, or (2) charge to accumulate on each photosensor in the array, respectively. The photosensor array can either capture the charge in a row-by-row fashion as in the latter case, or, alternatively, in the former case, the image can be captured as a whole (e.g., the photosensor array is exposed to a light source all at once). The processing of the charges which are stored on each of the photosensors is then performed either in a row-by-row fashion, or in a pixel-by-pixel fashion. Images captured in this row by row fashion is termed to be captured in a xe2x80x9cpipelinedxe2x80x9d mode of operation. For video image capture applications, a series of frames, or images, are captured similar to the manner above.
As digital cameras are positioned to replace traditional film-based cameras, they must be capable of operating under a variety of lighting situations. For example, digital cameras must be able to capture videos of scenes which are illuminated by sunlight, if outdoors, or which are illuminated by incandescent or discharge lights, if indoors.
However, when capturing a sequence of frames under an environment lighted by a discharge lamp (e.g., a fluorescent light), the digital video will contain artifacts due to the fact that the discharge lamp can vary in intensity and color temperature as a function of time. Thus, discharge lamps such as fluorescent lights or tungsten lamps do not offer a constant intensity of light but instead offer an intensity which, if measured and plotted on a chart, resembles a full wave rectified sine wave.
FIG. 1 shows an example of the intensity of fluorescent lighting as it varies over time, where the Y-axis represents the intensity of the light sensed by the photodiode, and the X-axis represents the passage of time. As can be seen by FIG. 1, the intensity of the light generated by fluorescent lighting (and thus sensed by the photodiode) is periodic and resembles the squared value of a sine wave. As the variation of the intensity is a function of time, a video stream that is captured in this lighting will include a potentially considerable amount of variation in the quality of the captured video as the capture is also a function of time.
The problem is also compounded in the fact that the variation in the intensity of discharge lamp lighting is different in different parts of the world as some countries use a 60 Hz alternating current (AC) power system and other countries use a 50 Hz AC power system. For example, the United States uses an AC power system which oscillates at 60 Hz. Thus, depending on the country in which the digital camera is used, the frame capture rate will have to be adjusted such that the frame capture rate is a function of the operating cycle of the power supply.
One approach that allows a digital camera to function under discharge lamp lighting of different power systems is to have the user enter a code designating the country in which the digital camera will be used. The camera would then adjust the frame capture rate according to the operating frequency of the country. The camera would maintain a list of correspondences between the regions in which it is currently operating and also what power system is functional in that region. Also, this approach will require the user to input a code every time a user entered a region with a different power system. Thus, this approach would require that the user manually enter the user""s current location.
A second approach would be to incorporate a system into the camera itself, such as a global positioning system (GPS), which would allow the camera to be xe2x80x9cself-awarexe2x80x9d as to which geographic location it is in and thus automatically sets the camera""s internal systems accordingly. However, this approach would require additional circuitry which would place additional power and cost requirements into the digital camera.
Yet another approach would be to include circuitry to have the camera automatically recognize the power system in which it is currently operating by having the user plug the camera into the power system of the country. For example, when a user reaches a certain location or a new region, the user would simply plug the camera into a wall outlet to allow the circuitry of a digital camera to register the operating cycle of the power system. However, this approach is also not recommended as different regions around the world usually have different configurations of wall sockets and connectors on those sockets such that the user would have to carry along a set of adapters, which could number into the tens or hundreds, to be sure that the user can plug the digital camera into the power system.
It would be preferable to have a system that minimizes the effects of using discharge lamp lighting that does not require user intervention or increased cost and power requirements on the camera itself.
What is disclosed is a method having the step of setting a programmable clock to a first frequency; capturing a first set of frames at the first frequency; and then determining a first standard deviation of a first set of average intensities for the first set of frames. In addition, the method also includes the steps of setting the programmable clock to a second frequency; capturing a second set of frames at the second frequency; and determining a second standard deviation of a second set of average intensities for the second set of frames. Then, comparing the first standard deviation with the second standard deviation; and, setting the programmable clock based on the comparison.